It is known in the prior art that ultrasonic energy may be coupled by liquid coupling techniques to the surface of an object being tested. This conventional type coupling, however, generally is considered not to be practical under circumstances that the object under test, such as a bloom or slab is at a high temperature.
It is also known in the prior art that electromagnetic acoustic (EMAT) and laser pulse generation techniques are two methods of noncontact generation and reception of ultrasound. These noncontact techniques are not without their drawbacks. For example, the EMAT method requires proximity of the transducer to the material being tested. The laser method requires high power and a confined beam to generate ultrasound. Further, both of these methods have poor sensitivity as receivers of ultrasound.
The use of air as a coupling medium has been discussed in several papers. One paper, entitled "Use of Piezoelectric Transducers for Contactless Ultrasonic Product Inspection", by V. I. Zaklyukoyskii and G. T. Kartsev, describes the use of a piezoelectric transducer for contactless ultrasonic inspection of objects of various materials and the use of matching components for coupling ultrasound into air. According to this paper the ultrasonic transducer uses a piezoelectric layer operating at half-wave resonance and one or more matching layers operating at quarter-wave resonance.
The invention distinguishes from the disclosure of the paper discussed above in that the ultrasonic transducer configuration of the invention uses a piezoelectric layer which operates at other than half-wave resonance and a first matching layer of a coupling medium which operates at other than quarter-wave resonance. Only a second matching layer of the coupling medium operates in quarter-wave resonance at the operating frequency determined by the composite of the transducer-first layer thickness and acoustic impedance. The net result is that the ultrasonic transducer of the invention has much higher efficiency than that described in the paper. The higher efficiency is supported by results of experiments in the aforementioned paper, which describe the capability of penetrating 3 cm of steel with a 10 to 1 signal to noise ratio (voltage or power not specified). According to the present invention penetration of steel of 10 cm and thicker, with a signal to noise ratio of 60 to 1 voltage (3600 to 1 power) has been obtained. In addition, the paper describes an operating frequency of 40-50 kHz. This frequency requires the use of acoustic waveguides to transmit the ultrasound to and from the test sample, and acoustic shields to prevent sound from leaking around the test sample. According to the invention, considerably higher efficiencies have been achieved with an operating frequency of 250 kHz, and even higher. This operation obviates the need for waveguides and shields.
Two other papers have some degree of relevance to the invention for reason of a reference to excitation of Lamb waves using airborne ultrasound. These papers are entitled "Metal Plate Testing Using Airborne Ultrasound", by M. Luukala and P. Merilainen and "Ultrasonics Plate Waves in Paper", by C. C. Habeger, R. W. Mann and G. A. Baum. According to the Luukala et al paper, the method of excitation and detection is carried out through use of well-known capacitive transmitters and receivers having, typically, an efficiency of about 0.2%. According to the Habeger et al paper, the method of excitation and detection is carried out with a transducer, such as a film transducer wherein the piezoelectric action is imparted to the film by repeatedly charging and discharging the capacitance of the film. The efficiency achieved by practice of the Habeger et al teaching is not disclosed, but there is no reason to believe that the efficiency achieved would be any greater than the efficiency achieved by the Luukala et al teaching.
The present invention, in contrast, provides for an efficiency of 7.0% or more using an ultrasonic transducer together with a piezoelectric layer and coupling medium, as generally described above and more particularly described below. The present invention, also, obviates various technical complexities which oftentimes are encountered with a laser-type system. In addition, the costs are considerably less. Further, the use of air to couple ultrasound poses no danger to an operator as with use of laser pulse ultrasound generation systems.